Não informado. (1957). Annual Bulletin of Gas Statistics for Europe. United Nations, Economic Commission for Europe.
Chicago-referens (17:e uppl.)Não informado. Annual Bulletin of Gas Statistics for Europe. Geneva, Suíça: United Nations, Economic Commission for Europe, 1957.
MLA-referens (8:e uppl.)Não informado. Annual Bulletin of Gas Statistics for Europe. United Nations, Economic Commission for Europe, 1957.
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